A large amount of carbon dioxide is generated from a thermal power generation equipment or a boiler equipment due to combustion of a fuel such as coal and heavy oil. In recent years, from the viewpoint of air pollution and global warming, there has been a worldwide movement of restricting an emission of carbon dioxide to the atmosphere. As one of CO2 separation/recovery technologies, a method of causing an absorbing solution containing an amine compound such as alkanolamine to absorb carbon dioxide, i.e. a so-called CO2 chemical absorption, has widely been known.
One example of power generation plants comprising a conventional CO2 chemical absorption system is shown in FIG. 8. The power generation plant comprises a boiler 1, a denitrification device 2, an air heater 3, an electric dust collection device 4, a wet desulfurization device 5, a prescrubber 10, a CO2 absorption column 20, a regeneration column 40, a reboiler 60, and so forth. A combustion exhaust gas discharged from the boiler 1 due to combustion of a fossil fuel such as coal is subjected to removal of nitrogen oxide in the denitrification device 2. After that, the exhaust gas is subjected to heat exchange in the air heater 3, followed by cooling to 120° C. to 170° C., for example. The exhaust gas after passing through the air heater 3 is subjected to removal of particulate matters in the electric dust collector 4. Subsequently, the exhaust gas is subjected to removal of sulfur oxide (SO2) in the wet desulfurization device 5 while increasing a pressure by an induced draft fan. Since about several tens of ppm of SO2 can sometimes remain in the outlet gas at the wet desulfurization device 5, the residual SO2 is minimized (e.g. to 10 ppm or less) by the prescrubber 10 disposed in front of the CO2 chemical absorption equipment to prevent the residual SO2 from deteriorating an absorbing solution circulating in the CO2 absorption column 20.
The CO2 absorption column 20 comprises a packing layer 21, an absorbing solution spray unit 22, a water washing unit 24, a water washing spray unit 25, a demister 26, a washing water reservoir 27, a cooler 28, a washing water pump 29, and so forth. CO2 contained in the exhaust gas is absorbed by the absorbing solution by gas-liquid contact in the packing layer 21 with the absorbing solution fed from an absorbing solution spray unit disposed at an upper part of the CO2 absorption column 20. Since the exhaust gas from which CO2 is removed (non-CO2 gas) is increased in temperature due to heat generated during the absorption reaction and is entrained with a mist, cooling and mist removal are performed in the water washing unit 24, and the mist is further removed in the demister 26 disposed above the water washing unit 24. After that, the exhaust gas is emitted as a treated gas 37 (non-CO2 gas). The washing water cooled in the cooler 28 is used circularly by the washing water pump 29.
The absorbing solution after absorbing CO2 (rich solution) is withdrawn from a reservoir at a lower part of the absorption column 20 by an absorption column withdrawal pump 33 and then is heated by a heat exchanger 34, and, subsequently, the rich solution is sent to the regeneration column 40. The rich solution is sprayed from a spray unit 42 inside the regeneration column 40 to be fed to a packing layer 41. Meanwhile, a vapor generated by the reboiler 60 installed at a lower part of the regeneration column 40 is fed to the packing layer 41. The rich solution and the vapor are brought into gas-liquid contact with each other in the packing layer 41, so that the CO2 gas is desorbed from the rich solution. Since the desorbed CO2 gas is entrained with an absorbing solution mist, mist removal is performed in a water washing unit 43 and a demister 45 installed above the water washing unit 43 to be discharged as a CO2 gas 46 from an upper part of the regeneration column 40. After that, the CO2 gas is cooled to about 40° C. by a cooler 47 and then is subjected to separation into a gas and drain water in a CO2 separator 48, so that the CO2 gas is introduced into a CO2 liquefying equipment (not shown) and the drain water is fed to the washing water spraying unit by a drain pump 50.
On the other hand, an absorbing solution from which CO2 is desorbed (lean solution) is stored in a regeneration column reservoir 51 and then is fed to the reboiler 60 through a reboiler solution feeding piping 52. A heat transfer tube and so forth are installed inside the reboiler 60 to heat the lean solution with a water vapor 62 fed to the heat transfer tube through a water vapor feeding piping. A vapor is generated inside the reboiler 60, and the vapor passes through a vapor feeding piping 65 to be fed to the regeneration column 40. The water vapor 62 used in the reboiler 60 becomes water inside the heat transfer tube to be recovered. The absorbing solution stored in the reservoir at the lower part of the regeneration column 40 is fed to a flash tank 91 through a regeneration column withdrawal piping 66. The absorbing solution is converted into a vapor by flash evaporation in the flash tank 91. The vapor is recompressed by a recompressor 92 and then fed to the regeneration column 40 to be reused as a heat source. A liquid phase part of the flash tank 91 is increased in pressure by a pump 93 and reduced in temperature by the heat exchanger 34 and the cooler 29 and then is fed to the CO2 absorption column 20.